1. Field of the Invention
The present invention relates to soles for shoes and more particularly relates to a midsole for an athletic shoe.
2. Description of Related Art
Soles in athletic shoes are expected to provide shock absorption and stability. Shock absorption minimizes the impact of a runner's footfalls to lessen stress on the leg muscles and joints. Stability is necessary to control the amount of lateral motion of a foot in order to prevent over pronation thereby lessen the stress on the lower legs.
During normal motion, the foot of a typical runner hits the ground heel first. The foot then rolls forwardly and inwardly over the ball of the foot. During the time that the foot is moving from heel strike to the ball of the foot, the foot is typically rolling from the outside or lateral side, to the inside or medial side of the foot; a process called pronation. After the ball contacts the ground, the foot continues rolling forward onto the toes. During motion through ball and toe contact, the foot rotates outward as the toes prepare to push off; a process called supination. The foot remains supinated while it is lifted off the ground between footfalls.
Pronation, the inward roll of the foot in contact with the ground, although normal, can be a potential source of foot and leg injury if it is excessive. Many prior art soles have been designed with the goal of preventing over pronation and controlling supination. The lateral motion of the foot, that is abduction and adduction, can be controlled by providing a stable sole. However, as the stability of the sole increases, the shock absorption properties of the sole decrease. Thus, soles must be designed to properly balance the properties of stability and shock absorption to provide optimum characteristics for both parameters. This design goal is further complicated by the fact that foot size is largely unrelated to body mass. For example, two people of equal weight may have feet that are two or three sizes apart and conversely, two people with the same foot size may have substantially different body mass. Thus, a shoe that is stable for a 130 pound, size 9 runner may not be stable for a 160 pound, size 9 runner.
Durability of the midsole, as measured by its ability to withstand cyclical loading without degradation of midsole properties, is also an important design goal. Most present-day athletic shoes use a midsole of an elastomeric foam, such as ethylene vinyl acetate (EVA). EVA foam allows designers to adjust the density, and hence the hardness, of the foam to provide various midsole properties in an attempt to balance shock absorption and stability. As is well-known to those skilled in the art, the higher-density EVAs provide a stable platform but less shock absorption, while the low-density EVA foams provide better shock absorption but less stability because they cannot control the lateral movement of the foot. EVA foams typically have a useful life of approximately 800,000 cycles before there is a noticeable degradation in their performance. For these and other reasons, there is a continuing search for alternative midsole designs.
Cohen, U.S. Pat. Nos. 4,753,021 and 4,754,559, discloses a midsole for a shoe having a sheet of rubber-like material with a plurality of ribs separating an upper and lower surface. As a load is applied to the midsole the ribs collapse thereby absorbing energy. As a load is removed the resilient nature of the ribs causes them to spring back to their previous shape. Cohen discloses plural embodiments including those in which the ribs form channels that are arranged parallel to, and orthogonal to a longitudinal axis of the elongate sole. Because of the design and choice of materials, Cohen would not represent an enhanced performance sole for use in an athletic shoe.